;;; The scopes of variables d,e, and f are the regions with the same background colors.

(define (quadirc-equation a b c)
	(if (zero? a)
		'error
		(let ((d (- (* b b) (* 4 a c))))
			(if (negative? d)
				'()
				(let ((e (/ b a -2)))
					(if (zero? d)
					(list e)
					(let ((f (/ (sqrt d) a 2)))
						(list (+ e f) (- e f)))))))))
